The search for a cost-effective and fuel efficient diesel aftertreatment solution is a significant challenge. Low-Emission Vehicle (LEV) tailpipe standards for diesel vehicles demand very low hydrocarbon (HC) and nitrogen oxide (NOx) standards for vehicles having 150,000 miles or more. Achieving and maintaining high efficiencies during EPA-mandated Federal Test Procedures (FTP) and Supplemental Federal Test Procedures (SFTP) may be difficult as typical operating conditions of on-road diesel vehicles readily degrade Pt NOx oxidation.
In attempts to meet the above-mentioned federal emissions regulations (among other standards and specifications) and reduce exhaust gas emissions, vehicle engines may be equipped with an emission control system comprising various emission control devices arranged in various configurations, such as three-way catalysts, diesel oxidation catalysts, particulate filters, NOx catalysts, and hydrocarbon traps.
One example configuration is shown by Choi in U.S. Pat. No. 8,168,125. Therein, a three-layer catalyst is located upstream of a selective catalytic reduction (SCR) device, and heat is provided by oxidizing hydrocarbons in order to release sulphur absorbed at an oxidation catalyst. Another example is shown by Chen et al. in U.S. Pat. 7,758,834. Therein, a three-way catalyst comprising only platinum group metals is supported on a cordierite support.
However, the inventors herein have identified potential issues with such systems related to stability of NOx oxidation over time under the typical operating conditions of on-road diesel vehicles. One potential issue may be that various functionalities of emissions control systems may compete and/or interfere with one another. For example, NO species may be oxidized to NO2 by a diesel oxidation catalyst upstream of a NOx catalyst after an engine cold start. However, the hydrocarbon and carbon monoxide oxidizing function of the diesel oxidation catalyst may interfere with the NOx oxidation function of the catalyst. As a result, even with modified configurations, emissions compliance, especially NOx emissions compliance, may not be achieved. This issue may be exacerbated as emissions regulations get more stringent while combustion processes become more efficient with significantly lower exhaust temperatures. As another example, the different configurations and the lower exhaust temperatures may complicate emission control device (ECD) temperature control strategies. As a further example, due to packaging volume constraints on the vehicle, the space available for the different configurations and functionalities of the emission control system may be limited. Overall, many current diesel oxidation catalyst and layered catalyst design systems may effectively worsen exhaust emissions or not provide lasting and robust emissions control.
In one embodiment, some of the above issues may be at least partially addressed by a layered emission control device (ECD) coupled to an engine exhaust, comprising: a first layer including a first oxidizing catalyst; a second layer including a hydrocarbon trap upstream from a third layer; and the third layer, comprising inner sublayers, each of the inner sublayers comprising base metal oxide (BMO) catalysts and having a BMO catalyst composition distinct from the other inner sublayers, wherein exhaust gas is directed into and emitted from the layered ECD at an upstream side of the first layer and at a downstream side of the third layer, respectively.
In another embodiment, a layered emission control system, including a layered emission control device (ECD) fluidly coupled to an engine exhaust may comprise: a first oxidizing catalyst layer positioned at an upstream side of the layered ECD within which a first portion of the exhaust gas is oxidized; a second oxidizing catalyst layer arranged at a downstream side of the layered ECD within which a second portion of the exhaust gas is oxidized; and a hydrocarbon trapping layer arranged intermediate to upstream side and the downstream side, wherein the second oxidizing catalyst layer comprises a plurality of second sublayers, each of the second sublayers comprise base metal oxide (BMO) catalysts, each of the second sublayers having a BMO catalyst composition distinct from the other second sublayers, and the first portion of the exhaust gas comprises one or more of hydrocarbons, CO, and SO2, and the second portion of the exhaust gas comprises NOx.
In a further embodiment, a method of treating exhaust gas from an engine, the engine including a layered emissions control device (ECD), may comprise: directing the exhaust gas at an inlet temperature to a first layer of the layered ECD positioned at an upstream side of the layered ECD, the first layer comprising a first oxidizing catalyst; oxidizing a first portion of the exhaust gas by the first oxidizing catalyst within the first layer; trapping a second portion of the exhaust gas within a second layer, the second layer including a hydrocarbon trapping material; directing the exhaust gas from the first and second layers to a third layer, the third layer including a plurality of inner sublayers, each of the sublayers comprising base metal oxide (BMO) catalysts and having a BMO catalyst composition distinct from the other inner sublayers; and oxidizing a third portion of the exhaust gas in the third layer, wherein the first portion comprises one or more of hydrocarbons, CO, and SO2, the second portion comprises hydrocarbons, and the third portion comprises NOx. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementation that solve any disadvantages noted above or in any part of this disclosure.